Dr. Anjan Kumar Chatterjee, presently the Chairman of Conmat
Technologies Private Limited, Kolkata, and concurrently the
Director of Dr. Fixit Institute of Structural Protection and
Rehabilitation, Mumbai, is in conversation with CE&CR.CECR: The cement industry is known to be a high emitter of CO2.
How can it be reduced? What are the cement manufacturers doing
to achieve low emission of CO2?
A.K Chatterjee: The carbon dioxide emission continues to remain a major
concern for the Portland cement manufacturing process. The concern
emanates from the release of about 535 kg CO2 per tonne of clinker
from calcination of limestone and about 330 kg CO2 per tonne of clinker
from combustion of fuel, resulting in direct emission of 835 kg CO2
per tonne of clinker. The corresponding figure for cement would vary,
depending on the quantity of clinker used in making a tonne of cement
and the grinding technology adopted. In this context, the industry at large
has adopted the following key levers to reduce the CO2 emission level:A. Clinker Making Stage
- Use of alternative non-carbonate calcium-rich raw materials.
- Enhancing use of alternative fuels in place of conventional coal.
- Making unit operations in the total manufacturing process more
energy efficient.
- Generating electricity with waste heat.
- Increasing the use of renewable energy.B. Cement Making Stage
- Use of supplementary cementing materials in substitution of clinker
in binary blended cements
- Manufacture of Portland ternary composite cements with even lower
clinker factor than the binary blended cements.
With all the above measures taken and initiated by the industry, the
present CO2 emission intensity is tentatively estimated at 0.7 t CO2/ t
cement, which was about 0.72 in 2010 and 1.12 in 1996. Although
this shows a significant reduction in CO2 emission during the last two
decades, the above levers may not be adequate to meet the future targets
committed and required for the climate change demand. It is projected
that the 2020 target for CO2 emission should be 0.58t/t and it should go
down further to 0.50t/t by 2050. These targets do not appear achievable
with the traditional levers like energy efficiency, use of biogenic fuels
and clinker substitution. Search for newer technological options for the
low carbon pathways is essential for the cement industry. Globally, the
following innovative research areas are receiving increasing attention:
- Reformulation of clinker composition
- Low carbon emitting manufacturing processes
- Carbon capture and use to produce carbonated cements and concrete
- Carbon capture and transformation into fuel.
The above areas are futuristic and have barriers of development from
concept proving to becoming viable processes. While some countries have
directed their resources quite substantially towards developing low carbon
pathways, the Indian industry is yet to take up the challenge of low carbon
process and product research in a mission mode. It is important to note
that the Indian cement industry is not only second in terms of production
volume in the world; it is also the most efficient in energy consumption
as compared to the world average. Further, improvements in the energy
and environmental parameters are expected through more extensive
use of alternative fuels and raw materials, and manufacture of Portland

cements with clinker factor of 0.50 or even less but still it is not enough,
and it is time for the industry to chalk out a blue print of more robust
and disruptive innovation and cooperative research on a national plane.CECR: How feasible is the production of low alkali Portland cement
in India, as our code of practice specifies this type of cement for
combating alkali aggregate reaction in concrete, especially in
concrete dams? What about the alkali by-pass systems? Are such
systems being used by the cement manufacturers?
A.K.C: It is true that in India the occurrence of reactive aggregates has
been noticed more widely than known previously. In fact, some regions,
which were taken for granted as sources of non-reactive aggregates,
are now seen to have pockets of reactive aggregates. Aggregates can
be reactive in most cases due to the presence of certain forms of silica
which result in alkali silica reaction (ASR). The types of silica that can take
part in ASR include strained varieties of quartz, tridymite, cristoballite
and amorphous forms like opals, flints, cherts, etc. In certain other
situations the aggregates can have a reactive form of dolomite that
causes ‘aggregate carbonates reaction (ACR)’.
In ASR, hydroxide ions in the pore solution react with reactive silica
present in the aggregates, resulting in internal stresses that can cause
expansion and cracking. Failure may occur within days or only after
years. The necessary conditions for ASR in Portland cement concrete
are a sufficiently high content of alkali oxides in the cement, a reactive
constituent in the aggregate and a supply of water. ASR is unlikely
to occur in concrete made with Portland cements, if the content of
equivalent Na2O (Na2O + 0.66 K2O) in the concrete is below 4 kg/m3
and a practical limit of 3 kg/m3 has generally been proposed to allow
for day-to-day variations in cement composition. The alternative
criterion based on cement composition (Na2O < 0.6%) does not allow
for varying cement content in concrete. It should also be borne in mind
that alkali cations may also be supplied from external sources including
mineral admixtures or aggregates. It may also be relevant to mention
that in great majority of cases, with use of composite cements with
supplementary cementing materials like fly as or granulated slag; the
ASR related expansion is sufficiently reduced to prevent failure of
concrete. Hence, the entire ASR related concrete failure issue must be
seen from various perspectives including the sources of input of alkalis
other than cement. However, the provision in our codes and standards
of testing the aggregate for its reactivity in large and critical projects
where concrete is likely to be exposed to humid atmosphere or wetting
action, is certainly an important safeguard but the recommendation of
using low alkali cement is only one option for preventing concrete failure.
Unlike ASR, in ACR the destructive expansion occurs in concretes
made with some aggregates containing dolomite, which reacts with
(OH) - ions in the so-called de dolomitization reaction, resulting in the
formation of CaCO3 and Mg(OH)2 or brucite. The expansion is mainly
caused by the growth of brucite crystals around the surfaces of the
dolomite grains. In this type of expansion and failure of concrete, the
alkali content is not directly responsible.
The manufacture of low alkali cement is obviously dependent on the
alkali contents in the raw materials used in making clinker. In preheated
kilns with suspension preheated cyclones and electrostatic separators,
volatiles present in the kilns practically have no escape route and the
circulating volatiles inside the kiln may often lead to entrapment of higher
amounts of alkali/sulphur in the clinker. This is partially overcome by
diverting a portion of the kiln exit gases with a bypass duct. A modern
bypass system consists of an air quench chamber, a shut-off valve, a
water-quench chamber and a separate dust collector. The installation
of a bypass system leads to additional energy and material losses. As
a thump rule one may indicate that with a 30% bypass system, the
fuel consumption increases by about 15% and the material losses by
about 7.5%. Technically speaking, in a preheated kiln provided with a
pre - calcination stage it is possible to produce low-alkali clinker from
high-alkali raw materials with more than 60% bypass, but its feasibility
is doubtful in most circumstances due to additional energy consumption,
problems of disposal of bypass dust and limited and sporadic market of
low alkali cement. The Indian cement industry is fully familiar with the
bypass technology and such facilities have been installed in a few plants
for tackling chlorine and sulphur problems, not so much for alkalis. It
must be understood that any bypass system is not selective for a single
volatile component the exit gases carry all the different volatiles present
in the system. Therefore, the degree of bypass required for different
components is different. In a practical situation the bypass is decided
upon for the most affecting components.
The above discourse is not intended to give an impression that the
low alkali cement cannot or is not made in India. It is made in selected
plants where there is availability of low alkali raw materials and where
the economics are favourable. Quite a few dams in India have been
constructed with indigenous low alkali cement. If required, it will be
produced under appropriate conditions. What might be helpful in future
planning is, to prepare an authentic demand projection for low alkali
cement in the country.CECR: The new BIS standard for composite cement has been issued.
What are the prospects of this cement for its utilization in concrete
construction?
A.K.C: It is certainly a very welcome development in India that we have
embarked on ternary composite cement specification with the adoption
of IS 16415:2015 with the following composition:
- Clinker/OPC (IS 16353): 35-65%
- Fly ash (IS 3812 Pt I): 15-35%
- Granulated slag (IS 12089): 20-50%.
It is important for two reasons – one, the clinker factor can be
reduced to a level that perhaps cannot be achieved in only fly ash-based
PPC; two, high particle packing density may be attained in concrete with
durability characteristics that are not normally encountered. There is also
a third advantage that the limitation of expanding the PSC production
due to limited availability of granulated slag may be overcome with this
ternary blend. Recognizing the likely benefits, the Indian cement industry
is engaged in commercializing the product. The product is expected in
the market any time now.
Notwithstanding these developments, it is important to note that the
composite cement specification in its present form is limited in scope
and options. The blending components are restricted to only fly ash
and slag, the latter being available in specific regions and the quantity
available is almost entirely tied up with different PSC producers. Hence,
the exploitation of the opportunity to produce new composite cement is
tilted in favour of properly located plants. Expanding the use of additional
blending components like calcined clay and limestone powder must be
considered in the revision of the standard, if wider adoption of multiblend
composite cement technology must be achieved in the country.
Further, the present standard specification is still prescriptive and not
performance oriented. There is a pressing need in the country to move
towards ‘performance oriented’ specification of cement to make use
of large variety of supplementary cementing materials. This only will
pave the way for resource conservation, environmental protection and
making concrete more durable. In addition to these aspects, one must
recognize that the concept of multi-blend composite cement is based on
the principle of high particle packing density, which is yet to sip into the
production and application professionals. The success of new composite
cements would depend on realizing the scientific and technological
concepts behind the products.CECR: Concrete code specifies PPC and PSC for aggressive
environmental conditions, e.g., the underground construction. In the
concrete piles and pile caps of bridges they should be used instead
of OPC. But the construction companies and Metro authorities have
preference for OPC. What are your views?
A.K.C: In simple terms, the concrete piles fall in two basic categories:
precast and cast-in-situ. Precast piles can be further divided into two
general classes: normally reinforced piles and pre-stressed piles. Castin-
situ piles are subdivided into piles with casing and piles without casing.
It is possible to have several variations of these basic types including
variation of cross-sectional area and longitudinal shape. Depending on
the foundation conditions and the type of concrete pile selected, the
load carrying ability of the pile can be developed either in skin friction
or point loading or a combination of the two. Concrete cast-in-situ
piles and more particularly pre-stressed concrete piles can sustain high
bending stresses and are frequently used in viaducts and trestle types
of structures with the pile extending above ground or channel bottom
level. From this very summary of pile types it is obvious that piling is a
specialized and project-specific engineering activity of fairly complex
nature and it may not be desirable to extend the conventional concept
of choosing a particular type of cement, based on broad environmental
considerations.
Durability of piles is generally ensured by the composition and density
of concrete, use of sound and hard aggregates, and designing proper
concrete cover over the reinforcing steel. Often the concrete mix is
rich and proper care is taken in mixing, placing, consolidating and curing
to achieve hard dense concrete. Piles can be protected against some
of the agents of deterioration by use of coatings and jackets applied
to vulnerable areas. Plain or reinforced concrete piles embedded in
earth are generally considered not subject to deterioration. The water
table, if free from deleterious substances, does not affect durability.
In extremely infrequent situations there are possibilities of permeation
and damage by ground water saturated with either acids or alkalis or
salts. Dense rich concrete with sulphate-resisting cement is a means of
minimizing the effect of a deleterious environment. It may also be kept
in mind that for precast and pre-stressed types of piles, use of blended
cements is not feasible.
Notwithstanding what has been stated above in the context of pile
foundation, the benefits of using blended cements or supplementary
cementing materials in concrete for placement in adverse environmental
conditions cannot be ignored. The example of the construction of
Confederation Bridge across the Northumberland Strait between New
Brunswick and Prince Edward Island in Canada way back in mid-nineties
may prove the point. To withstand a very aggressive marine environment
there including freezing/thawing a concrete was designed with 480
kg/m3 total cementitious content, of which 10% was a class F fly ash
and 6.75% was silica fume. The water-cementitious materials ratio was
less than 0.30. The average 90-days strength of the concrete was 80
MPa against the structural requirement of 60 MPa with an air content
of 6-8%. The above prescription of concrete served quite well in the
aggressive marine environment. This example will perhaps bring forth
the fact that the design of concrete is at the core of its durability in
aggressive environments including those of the pile foundation. Use of
blended cements is not ruled out but its application must be accompanied
with material knowledge and good practices.

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